You guys was so fast that dont even delete the comment, epic

hi,
tried it on godbolt,
the resulting assembly-code will be the same,
so there's no real difference.
From my point of view it's just a habit to mark
any cast with the available keywords..
even in katas, it just grew into my muscle memory.

Source of the habit might be this: "https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines" (chapter ES.49) some but not all went in our companies codestyle-guide, so i'm just used to it.

What is the difference between this and return i?

This solution is incorrect for tree with root = -1 and only one leaf = -2: it will return -1 instead of -3.

Thank you! I'm very pleased :))

Using a primes cache accelerates the iteration.

Well .. Nice .. Very nice Code ... O(logn) is the ideal Complexity for this Problem .. The Code is Short yet Effective .. Bravo and BTW ...if it's not personal or vulgar to quest.. you have Impressive Skills in C++ , and Good Understanding To the Effecient Algorithms (i.e) why Don't Post/add Katas ?? ... Regards .. Zizou